Ataques de oportunidade não exigem movimento para e Frightened não impede uma criatura de rolar ataques de oportunidade, em desvantagem.
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
A frightened creature has disadvantage on Ability Checks and Attack rolls while the source of its fear is within line of sight. The creature can’t willingly move closer to the source of its fear.